An atom is the smallest unit of a chemical element that retains all of its physical and chemical properties.

  • At the core of an atom is a nucleus that contains protons which are positively charged sub-atomic particles. The number of protons defines the atomic number and thus the chemical element of the atom. For instance, a hydrogen atom has one proton.
  • In addition to protons, the nucleus of an atom also houses neutrons, sub-atomic particles with a mass slightly larger than protons but with no electrical charge.
  • Circling the nucleus are negatively charged particles called electrons, which are kept in place by their attraction to the positively charged protons in the nucleus.
  • In a neutral atom, the quantity of electrons equals the number of protons.
  • A neutral atom is an atom where the number of protons, which are positively charged, equals the number of electrons, which are negatively charged.
  • When an electron is gained or lost from an atom, it forms a charged particle known as an ion.
    • A cation is a positively charged ion resulting from the loss of electrons, making the number of electrons fewer than the protons.
    • An anion is a negatively charged ion resulting from the gain of electrons, making the number of electrons more than the protons.
  • The total number of protons and neutrons in an atom’s nucleus determines its atomic mass.
  • Atomic mass measures the total mass of protons and neutrons in an atom and helps in the arrangement of elements in the periodic table.
  • In atomic theory and quantum mechanics, an atomic orbital is a mathematical function that describes the behaviour and location of an electron in an atom’s electron cloud.